Acid gases are present as impurities in numerous industrial fluids, i.e., liquid and gas streams. These acid gases include hydrogen halides such as HCl, HF, HBr, HI and mixtures thereof. For example, one of the key processes in refining petroleum is catalytic reforming. In the catalytic reforming process, a light petroleum distillate or naphtha range material is passed over a noble metal catalyst to produce a high octane product. Hydrogen is a by-product of the catalytic reforming process, and a portion of the by-product hydrogen is recycled to the reaction zone to maintain catalyst stability. Typically, the noble metal reforming catalyst is promoted with chloride which, in the presence of hydrogen, results in the production of a small amount of hydrogen chloride. Thus, the net by-product hydrogen withdrawn from the catalytic reforming process generally contains a small amount of hydrogen chloride. Similarly, in a process for the dehydrogenation of light isoparaffins to produce isoolefins, the promoting of the noble metal catalyst with chloride will produce a net hydrogen stream containing small amounts of HCl. The net hydrogen produced in the catalytic reforming process and the dehydrogenation process is generally used in sensitive downstream catalytic processes. In addition, there are other hydrocarbon and chemical processes in which small amounts of HCl are generated and carried away in gas or liquid streams. Even small amounts of gaseous HCl present in the net hydrogen can seriously interfere with the operation of downstream processes which use the hydrogen and can cause corrosion problems in the equipment such as pipes, valves, and compressors which convey hydrogen. Generally, HCl in gas or liquid hydrocarbon streams must be removed from such streams to prevent unwanted catalytic reactions and corrosion to process equipment. Furthermore, HCl is considered a hazardous material and releasing the HCl to the environment must be avoided.
Currently, activated alumina is the most widely used sorbent in the petroleum refining and chemical industries. Activated alumina is employed as a scavenger for the removal of small quantities of HCl from fluid streams. Significant developments to improve the performance of alumina to remove HCl from hydrocarbon streams are disclosed in U.S. Pat. Nos. 4,639,259 and 4,762,537 which relate to the use of alumina-based sorbents for removing HCl from gas streams. U.S. Pat. Nos. 5,505,926 and 5,316,998 disclose a promoted alumina sorbent for removing HCl from liquid streams by incorporating an alkali metal oxide such as sodium in excess of 5% by weight on to an activated alumina base. It is also known that alumina can be promoted to sorb more HCl by impregnating the alumina with sodium carbonate or sodium hydroxide or calcium hydroxide. U.S. Pat. No. 4,639,259 discloses the use of calcium acetate to improve the dispersion of the calcium oxide on the alumina to achieve higher sorption capacity. The use of promoted alumina compared to other alumina-based sorbents can extend the length of time a fixed amount of sorbent will sorb HCl. By increasing the content of promoters such as sodium carbonate or sodium hydroxide, the HCl sorption capacity of the scavenger can be increased. However, the addition of promoters to alumina to improve the capacity of the sorbent for HCL appears to have a point of diminishing returns. Despite the type and amount of promoter incorporated into the alumina-based and promoted alumina materials, commercial experience shows that alumina-based and promoted alumina sorbents have a relatively low capacity for the sorption of HCl, often limited to levels less than 10-16 wt-% HCl.
Existing sorption processes for removing HCl from hydrocarbon-containing streams typically involve passing the hydrocarbon-containing fluid stream over the sorbent, which is disposed in a fixed bed. Conventionally, these fixed beds contain alumina-based sorbents wherein sodium or calcium is doped or coated on the alumina. Typically, the alumina-based and promoted alumina materials are formed into nodules or spheres. As the alumina-based sorbents pick up HCl, the sodium or calcium promotor, as well as aluminum, reacts with HCl to form chloride salts. Because HCl molecules are able to form hydrogen bonds with chloride ions, a limited amount of HCl can become physically sorbed on the surface of the salt molecules. However, the alumina sorbent in this service is known to have the undesirable property of converting certain hydrocarbons in the streams into a substance often called "green oil" which often collects in the fixed sorbent bed. Typically, these green oils are green or red in color. They generally contain chlorinated C.sub.6 -C.sub.18 hydrocarbons and are believed to be oligomers of light olefinic hydrocarbons. The presence of green oils in the fixed sorbent bed fouls the sorbent bed and results in the premature failure of the sorbent. When this fouling occurs, often costly measures are required to remove the spent sorbent from the bed. Furthermore, the chloride content of the green oils on the spent sorbent makes disposal of the spent sorbent an environmental problem. While the exact mechanism of green oil formation is unknown, it is believed that green oils are formed by catalytic reaction of aluminum chloride or HCl with the hydrocarbon resulting in a chlorinated hydrocarbon. Since both aluminum chloride and free HCl are known to be acidic and present on the surface of the sorbent, they are able to catalyze the polymerization of reactive hydrocarbons. Since it is very difficult to avoid the physical sorption of HCl and the formation of chloride salts on alumina-based and promoted alumina sorbents, the catalyzed polymerization reaction of hydrocarbon and the formation of green oil is not easily avoided. Green oil formation remains an unresolved industry problem during the removal of HCl from hydrocarbon streams.
When unsaturated hydrocarbons such as butadiene or other olefinic compounds are present in a hydrocarbon-containing stream, these compounds can be polymerized on acidic surfaces. Alumina based sorbents and promoted alumina sorbents, once they adsorb HCl, become acidic during the sorption process, and thus, acquire catalytic activity for the polymerization of the reactive hydrocarbons in the stream. When green oils are produced during the HCl sorption process, the spent sorbent represents a costly disposal problem. The formation of these polymers fouls the sorbers, shortens sorbent life, and creates a problem for the disposal of the solid adsorbents now containing chlorinated hydrocarbons. Since an HCl sorbent is not regenerable, the treatment of streams with even moderate to high HCl content, such as an HCl sorbent with a capacity of 10-16 wt-%, requires the fixed bed of sorbent to be changed frequently and imposes a downtime on the upstream process. Because the change of sorbent beds containing polymerized hydrocarbons requires costly measures to dig the sorbent out of the sorbent bed, the loss of production time and the maintenance costs are especially significant. The polymerization or acidic reactivity of the Cl loaded adsorbents must be reduced to avoid these problems.
There are many compounds that are reactive to acid gases such as hydrogen halides which can be employed as a scavenger sorbent to remove trace amounts of acid gases from fluid streams. However, for a compound to function in a fluid stream from a process plant where hydrocarbons are present, the material must have good acid gas sorption capacity, have sufficient physical strength, and be catalytically inert in the presence of reactive hydrocarbons. That is, the compound should have a reduced catalytic activity. By a reduced catalytic activity, it is meant that the catalytic activity is about 1/3 to 1/2 that of the catalytic activity of the conventional sodium promoted alumina.
It is an objective of the present invention to provide a sorbent which is effective for removing HCl from hydrocarbon streams and which is catalytically inert to reaction of those hydrocarbons to form the green oils.
It is an objective of the present invention to provide a sorbent for removing HCl from a hydrocarbon stream with an improved capacity for sorption of HCl with a minimum requirement for maintenance costs.
It is objective of this present invention to provide a sorbent which avoids the production of potentially hazardous chlorinated hydrocarbons.